1. Field of the Invention
The present invention relates to a radar data selection equipment, and, more particularly, to an improved radar data selection equipment suited for use in an aviation control system.
2. Description of the Prior Art
Most of the aviation control systems in use in Japan are operated through wireless communications. Even in an aviation control system using radar, an air traffic controller judges aviation conditions to maintain safety by confirming the position of an airplane according to (1) a strip obtained through flight data processing (FDP) based on flight schedule data supplied from the pilot of the airplane before its takeoff and according to (2) a plane positional data detected by the radar. Such a human-controlled system has become obsolete with the recent rapid increase in the volume of air traffic. Unfortunately, a number of accidents have occured due to the failure of conventional aviation control.
Under the circumstances, research and development of an air-route automatic control system are in progress to permit automatic aviation control by (1) monitoring air traffic over the entire domestic area through a plurality of radar systems and by (2) processing the monitored data by an electric computer system. The heart of such an aviation control system is the radar data processing system (RDP) which is provided with a radar target detector as it is sometimes called, or digitizer.
FIG. 1 is a block diagram showing an example of the digitizer which consists essentially of a target detecting and processing part 1, and a buffer memory 2. The target detecting and processing part 1 comprises a signal detecting part and a correlation computing part. The signal detecting part is one in which analog data from the primary radar and the secondary radar are quantized into digital signals. The correlation computing part computes the correlation between the primary and secondary radar data by a statistical processing method, such as the sliding window method, to obtain positional data of a target, i.e., an airplane, expressed in terms of distance and direction angle or data such as identification code and flight altitude. The buffer memory 2 stores digital data supplied from the target detecting and processing part 1. The output of the digitizer is transmitted to a target follow-up computer 3 through a transmission channel 4. A suitable display device such as a cathode-ray tube is used to display the target data in a digital form together with the description indicated by the side thereof. This display enables the control officer to view data on an airplane within a range of about 300 km in radius, with respect to position, flight direction, identification, aircraft number and other data.
The primary radar PSR refers to an ordinary radar capable of receiving a signal reflected from a target, i.e., an airplane. The secondary radar SSR is one which sends an interrogation signal to a flying airplane by way of an antenna rotated coaxially with the antenna of the primary radar, thereby interrogating the airplane identification (such as the name and number of the airplane) or flight altitude. The airplane, when equipped with a transponder, makes an automatic response, in terms of an electric waveform, to the interrogation.
Generally, the signal received by the primary radar contains strong, unnecessary waves, called clutter signal, reflected from clouds, mountains, and other objects. The clutter signal causes the buffer memory of the digitizer to overflow leaving no capacity to accept useful data. In such an event, a target cannot be identified as true data. One approach to this problem is to provide the buffer memory with a sufficient capacity. With this improvement, however, the quantity of data supplied to the follow-up computer must be within the capacity of the computer. To this end, it is necessary to secure the target data by causing the useless clutter data to overflow when the memory becomes full.
The target data detected by the digitizer of the radar data processing system contains (1) data on a clutter signal and an airplane which have been received by the primary radar, and (2) data on an airplane, which has been received by the secondary radar. The clutter signal comprises unnecessary data produced due to radar reflection from clouds, waves on the sea, mountains and other causes. This clutter data accounts for a considerable part of the detected data from the primary radar. Most of the detected data from the secondary radar is useful and equivalent to the data detected by the primary radar. Therefore, in order to effectively prevent the useful data from being wasted due to memory overflow, the primary radar data which contains a considerable amount of useless data must selectively be discarded.